The present invention relates to a process by which a printed circuit board having a high-density wiring pattern can be produced efficiently in a short period of time.
A conventional process for producing a printed circuit board comprises the following steps: cutting necessary holes through a copper clad laminate and plating the walls of the through-holes; applying an etching resist over the areas where a circuit pattern is to be made; performing exposure and development of the resist; and stripping the unnecessary portions of the resist. More recently, in order to cope with the demand for increasing the density of circuit patterns to be produced, attempts have been made to pattern etching resists by a photomechanical process. There are two types of photoresist that are used as etching resists in a photomechnical process, and they are negative and positive. A negative photoresist becomes less soluble in the solvent (developer) in exposed areas relative to unexposed areas. The solvent-soluble component in a negative photoresist undergoes cross-linking by absorbing the energy of actinic radiation and thus becomes insoluble in the solvent. The cross-linked and insoluble portions of the resist are still prone to swelling in the solvent and hence can cause a problem when a high-density wiring pattern is to be obtained from the patterned photoresist. Another problem with negative resists is their vulnerability to dirt or dust particles on the mask or flaws in its surface; the area affected with these troubles remains unexposed and by subsequent development and etching, the wiring pattern in that area will become interrupted. Interruptions of the wiring pattern may also occur in through-holes which are inaccessible by actinic radiation. A positive resist is made of a polymer that is inherently insoluble in the solvent and which becomes soluble upon exposure to actinic radiation. Because of this nature, a positive resist will not swell during development with the solvent, which is a great advantage for the purpose of producing a fineline pattern. Even if dirt or dust particles of surface flaws prevent exposure, the unexposed area will remain intact during subsequent development and etching and may be retouched by another cycle of etching. The problem with inaccessible through-holes is also absent from a positive resist.
A photomechanical process involves the step of preparing a negative mask which is complementary to the pattern of etching resist to be produced and this preliminary step has been one of the causes of inefficient fabrication of printed circuit boards.
Applications of a positive photoresist to the manufacture of printed circuit boards are described in such references as Printed Circuit World Convention III, Technical Paper, 23 (1984), Printed Circuit World Convention IV, 62 (1987), and Nikkei New Materials, Jan. 5, 1987, pp. 10-11.
Methods of resist patterning by scanning with laser light are described in such references as Electronics, 32 (11), 17-19 (1986), Electron Pack. Prod., 25 (6), 64-68 (1985), and Printed Circuit World Convention IV, Technical Paper, 22 (1987).
Cases in which electron beams or eximer laser light are used as light sources for exposing photoresists in the fabrication of printed circuit boards are described in such references as Laser Kagaku Kenkyu (Study of Laser Science), No. 8, 111 (1986), and Nikkei Microdevices, February , 1988, 115.
Negative masks have been necessary when conventional positive resists are used. When a laser beam is used as exposing light for patterning, a negative resist must be employed but this is not suitable for the purpose of fabricating printed circuit boards with a high-density wiring pattern within a short period of time. Conventional negative and positive resists are sensitive to ordinary light and must be handled under safety light. Not only does this require an extra capital investment but also the safety light which is yellow or red increases the burden on the operating personnel.
Electron beams are commonly used to cure paint films. When electron beams are used as a light source for exposing photoresists, they have a sufficient energy to cut the backbone chain of vinyl polymers, thereby forming positive resists from those polymers. Another advantage of electron beams is their scanning capability. An eximer laser is another source of light that has a sufficient energy to cleave the backbone chain of vinyl polymers. It is also capable of scanning the substrate during exposure by moving the substrate carrying stage for successive shots. Both electron beams and eximer laser light are actinic radiations that have short wavelengths, so that materials that are sensitive to these kinds of light can be handled under ordinary light. However, the resists that have conventionally been used in the step of exposure in the fabrication of printed circuit boards using either electron beams or eximer laser beams as exposing light are of a negative acting type and hence are not suitable for the purpose of producing a fineline pattern.